Squeeze tubes made of thermoplastic are commonly used for containing and dispensing various substances from caulk to food products to health and beauty aids. The majority of squeeze tubes currently on the market are made by injection-molding the head portion of the tube that defines the nozzle or orifice from which the contents will be dispensed, separately extruding the tubular body portion of the tube, and then joining the head and body portions together by a suitable process such as fusion bonding or welding. For economic and other reasons, it would be advantageous to injection-mold the entire tube as a one-piece integral part.
Injection-molding of squeeze tubes, however, poses a number of challenges. The tube must be soft and pliable so that it can readily be compressed by hand to dispense the contents, and hence the wall of the tube must be relatively thin and the modulus of the thermoplastic material must be relatively low. The thin wall, coupled with the substantial length required of some types of squeeze tubes, can result in a large length-to-thickness ratio for the tube (for example, greater than 250, or even greater than 400). The large length-to-thickness ratio makes molding difficult because the molding composition is resistant to being forced through the long, thin mold passage; high injection pressures must be used, and the molding composition must have a low viscosity. Even with a low-viscosity molding composition, the requisite high injection pressure can cause the core of the mold to deflect to one side, particularly if the core is supported at only one end as is the case when molding a tube to have a closed head end. Such core deflections, if large enough, can lead to unacceptably high wall thickness variations in the finished tube.
Polyolefins such as low-density polyethylenes are available that are soft enough to make a readily compressible squeeze tube, but the tube must also be resistant to cracking when compressed. Polyethylene is prone to environmental stress cracking when exposed to certain types of substances, particularly many health and beauty products, and hence cannot be used as the sole ingredient in a molding composition for squeeze tubes that are to contain such products. Tear-resistance is also an important requirement for a squeeze tube.
U.S. Pat. No. 6,124,008 describes a composition for injection-molding of squeeze tubes that is said to substantially improve the environmental stress-crack resistance (ESCR) of the tubes. The composition comprises a first ethylene/C4 to C5 olefin copolymer such as linear low-density ethylene/butene copolymer, blended with a second ethylene/C6 to C10 olefin copolymer such as linear low-density ethylene/octene copolymer. The melt index (MI) of the first component is given as about 10 to 20 g/10 minutes, and the MI of the second component is given as about 4 to 8 g/10 minutes. It is believed such a composition would not be suitable for injection-molding of squeeze tubes having a very high length-to-thickness ratio (e.g., greater than 250) and a substantially uniform wall thickness (e.g., uniform to within ±0.005 inch) around the circumference and along the length of the tube, particularly in the case of closed-end tubes molded with the mold core supported at only one end, where the core is especially susceptible to being deflected by the high injection pressures required.